03-09-00
CORVALLIS, Ore. -- Researchers say in a new report that if a huge asteroid were to hit the Earth, the catastrophic destruction it causes, and even the "impact winter" that follows, might only be a prelude to a different, but very deadly phase that starts later on.
They're calling it, "ultraviolet spring."
In an analysis of the secondary ecological repercussions of a major asteroid impact, scientists from Oregon State University and the British Antarctic Survey have outlined some of the residual effects of ozone depletion, acid rain and increased levels of harmful ultraviolet radiation. The results were just published in the journal Ecology Letters.
The findings are frightening. As a number of popular movies have illustrated in recent years, a big asteroid or comet impact would in fact produce enormous devastation, huge tidal waves, and a global dust cloud that would block the sun and choke the planet in icy, winter-like conditions for months. Many experts believe such conditions existed on Earth following an impact around the Cretaceous-Tertiary, or K-T boundary, when there was a massive extinction of many animals, including the dinosaurs.
That's pretty bad. But according to Andrew Blaustein, a professor of zoology at Oregon State University, there's more to the story.
"Scientists have pretty well documented the immediate destruction of an asteroid impact and even the impact winter which its dust cloud would create," Blaustein said. "But our study suggests that's just the beginning of the ecological disaster, not the end of it."
Blaustein and colleague Charles Cockell examined an asteroid impact of a magnitude similar to the one that occurred around the K-T boundary, which is believed to have hit off the Yucatan Peninsula with a force of almost one trillion megatons.
The immediate results would be catastrophic destruction and an impact winter, with widespread death of plants and the large terrestrial animals -- including humans -- that most directly depend on those plants for food. That's the beginning of an ugly scenario, the researchers say.
As a result of the impact, the atmosphere would become loaded with nitric oxide, causing massive amounts of acid rain. As they become acidified, the lakes and rivers would have reduced amounts of dissolved organic carbons, which would allow much greater penetration of ultraviolet light.
At first, of course, the ultraviolet rays would be blocked by the dust cloud, which sets the stage for a greater disaster later on. Many animals depend on some exposure to ultraviolet light to keep operational their biological protective mechanisms against it -- without any such light, those protective mechanisms would be eroded or lost.
During the extended winter, animals across the biological spectrum would become weaker, starved and more vulnerable. Many would die. Then comes ultraviolet spring, shining down on surviving plants and animals that have lost their resistance to ultraviolet radiation and penetrating more deeply, with greater intensity, into shallow waters than it ever has before.
"By our calculations, the dust cloud would shield the Earth from ultraviolet light for an extended period, with it taking about 390 days after impact before enough dust settled that there would be an ultraviolet level equal to before the impact. After that, the ozone depletion would cause levels of ultraviolet radiation to at least double, about 600 days after impact."
According to their study, these factors would lead to ultraviolet-related DNA damage about 1,000 times higher than normal, and general ultraviolet damage to plants about 500 times higher than normal. Ultraviolet radiation can cause mutations, cancer, and cataracts. It can kill plants or slow their growth, suppressing the photosynthesis which forms the base of the world's food chain.
Smaller asteroid impacts, which have happened far more frequently in Earth's history, theoretically might cause similar or even worse problems with ultraviolet exposure, the researchers say. The ozone depletion would be less, but there would also be less of a protective dust cloud.
"Part of what we're trying to stress here is that with an asteroid collision, there will be many synergistic effects on the environment that go far beyond the initial impact," said Cockell, a researcher with the British Antarctic Survey who did some of this analysis while formerly working with NASA. "Effects such as acid rain, fires, the dust clouds, cold temperatures, ozone depletion and ultraviolet radiation could all build upon each other."
During the K-T event, the scientists said, many of the animals may actually have been spared most of the ultraviolet spring they envision. That impact, oddly enough, hit a portion of the Earth's crust that was rich in anhydrite rocks. This produced a 12-year sulfate haze that blocked much of the ultraviolet radiation. But it was a lucky shot -- that type of rock covers less than 1 percent of the Earth's surface.
So when the next "big one" comes, the scientists said, the ecological repercussions may be more savage than any of those known in Earth's long history. The collision will be devastating, the "impact winter" deadly.
But it will be the ultraviolet spring that helps finish off the survivors.
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